This invention relates to the electrolytic recovery of nickel and zinc from solutions thereof such as in conventional electrowinning processes.
In such electrolytic processes the solution from which the metal is recovered generally includes the anions of the acid used to leach the metals into solution and in order to provide an economic process this acid is preferably, if not of necessity, regenerated in the electrolytic cell. In order for this to be achieved the oxidation potential of the said anions should be higher than the decomposition potential of water at normal operating conditions in order to avoid oxidation of the anions and thus destruction of the capability of regenerate the leaching acid.
It is for the above reason that nickel and zinc are generally leached using sulphuric acid since the sulphate ion has a high oxidation potential and thus the hydrolysis of water takes place at the anode in preference to the oxidation of the sulphate ions.
It has long been recognized from the general theoretical point of view that hydrochloric acid has more desirable properties in particular higher conductivity but the chloride anion would, in a conventional electrolytic cell be lost as a result of being oxidized to form chlorine gas at the anode. Thus, the process would be uneconomical in view of the high cost of hydrochloric acid quite apart from the dificulties created by the chlorine gas evolved at the anode.
The latter problems are clearly indicated in U.S. Pat. Nos. 2,578,839 and 2,480,771 to Renzoni wherein a special three compartment type of cell is provided simply to enable sulphate type of electrolytes containing relatively small amounts of chloride ions to be electrolized to recover nickel. These patents disclose cells wherein distinct anode compartments and cathode compartments are separated by middle compartments. In each case the compartments are defined by fabric type diaphragms and sulphuric acid anolyte is induced by means of a suitable head, to flow through the diaphragms defining the anode compartments into the middle compartments to prevent chloride ions reaching the anodes. Since these cells regenerate sulphuric acid the flow of anolyte through the diaphragm does not affect the re-usable characteristics of the spent electrolyte which is basically sulphate in nature. These patents further illustrate the high power consumption associated with a sulphate system -- the FIGURES being given in the preferred example as 6.5 volts to produce a current density of 0.033 amperes per square centimeter. Owing to the permeable nature of the diaphragms used to define the anode compartments such cell would be valueless in the electrolytic recovery of nickel from substantially pure chloride solutions owing to contamination of the regenerated acid with sulphuric acid from the anode compartments.
On the other hand, whilst not relating to the recovery of nickel or zinc, U.S. Pat. No. 3,072,545 to Juda et al. describes a similar cell for use in regenerating spent pickle liquors and wherein the separate anode compartment is utilized to prevent the oxidation of oxidizable cations at the anode. In this case the anode compartments are defined by ion exchange diaphragms which positively prevent the flow of anolyte through the diaphragms. Such a cell is also unapplicable to the economic recovery of metals since the ion exchange diaphragms exhibit high electrical resistance with a corresponding high power consumption.